In a number of devices, such as radiation imagers, it is desirable to apply a thin membrane to an irregularly shaped surface. For example, in a radiation imager in which a scintillator is optically coupled to a photosensor, it is desirable to apply a layer of reflective material to the surface of the scintillator opposite the surface adjoining the photodetector so that optical photons generated in the scintillator are reflected towards the photosensor. Commonly, one surface of the scintillator is irregularly shaped, that is, protrusions extend from the surface so that the surface is not flat. Such protrusions are needle or pyramid-like structures that result from the deposition process and serve to localize detection of photons generated in the array to the area in which the incident radiation was absorbed in the scintillator.
Application of reflective coatings to the irregular surface of the scintillator poses a number of difficulties. Many scintillator materials, such as cesium iodide, have large thermal expansion coefficients and thus are extremely sensitive to processes in which a reflective coating is deposited onto the surface, such as by sputtering. The relatively high temperatures associated with such deposition processes (e.g., above about 300.degree.-400.degree. C.) cause degradation of the scintillator material that and the overlying reflective material, resulting in much degraded performance of the scintillator. Traditional techniques of mechanically applying a coating, such as pulling a coating sheet across the surface to be covered, or mechanically pressing a coating onto the surface from above, cannot be used because of the malleable nature of cesium iodide. Specifically, the cesium iodide needle structure (on the light-receiving side of the scintillator) is deformed by such conventional processes, resulting in a distortion of the needle structure, causing light traveling through the structure to strike the interior surface boundaries of the needle many thousand more times before the right emerges from the scintillator into the detector array. Deformation of the needle structure as results from conventional cover application processes result in degraded imager performance (as measured, for example, by the imager's modulation transfer function (MTF)). Further, most conventional mechanical cover application processes do not provide for the evacuation of air that may be trapped between the scintillator surface and the cover material; such trapped air further degrades the optical performance of the scintillator.
A reflective coating on a scintillator surface desirably conforms to the irregular shape of the scintillator so that optical photons are directly coupled between the reflective layer and the scintillator material, with few it any interstitial voids between the scintillator material and the reflective material. Additionally, the application of the reflective material should not degrade the scintillator structure, either by thermally degrading the material or mechanically deforming the needle or pyramid-like structure of the scintillator. Reflective materials having the desired optical and physical characteristics are available in monolithic thin membranes (as used herein, "monolithic" refers to a substantially uniform material in a sheet-like form), however application of such thin membranes to the irregular shaped surfaces of a scintillator without damaging the scintillator or the thin membrane has been problematic.
One object of this invention is to provide a method for applying a thin membrane over a scintillator on a radiation imager, including precisely aligning the membrane with the surface and causing the membrane to be conformingly disposed over the needle-like structures of the scintillator without causing deformation or lateral movement of the needle-like structures.